Jet-propelled small-sized boats of the type in which a single person is allowed to ride the boat in a standing posture on the floor deck at the stern while steering the boat for gliding freely over water surface have already been known, as found, for example, in Japanese Patent Publication Nos. 12114/1978, 12115/1978, 21197/1978, and 21197/1978 and Japanese Utility Model Publication No. 41197/1984. However, in each of these known boats, the propeller shaft for rotatively driving the propeller by the engine mounted on the boat is in the form of a simple, solid metallic round bar, which is unsuitable from the standpoint of the object of making the boat lightweight and increasing buoyancy.
It can be said that it is effective in achieving the above object to make most of the lengthwise intermediate portion of the propeller shaft in hollow construction, provided that the rotative power of the engine can be properly transmitted to the propeller shaft and that the required strength and stability of the bearing regions of the propeller shaft can retained.